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When an element is needed for styling purposes or as a convenience for scripting
-- A container div is normally intended to be a scripting convenience/ styling purposes.

A general rule is that the section element is appropriate only if the element's contents would be listed explicitly in the document's outline.
-- The container/root level div isn't normally important to the hierarchy of the document.

For:

generic document or application section
-- Theoretically it could be considered the root level view/app section

A section, in this context, is a thematic grouping of content, typically with a heading, possibly with a footer.
-- It could be considered a thematic grouping of primary content/sections and normally includes some type of heading and footer.

With that said, I believe the argument against is more solid and less subjective then for. So I vote for sticking to div if its absolutely needed. The primary purpose of a root level container is mostly presentational then anything else.

You don't need to, but it might be helpful for users with a browser/AT combination that supports <nav> (though I'm not aware of any that support it yet). It might also be useful for search engines to be able to identify your navigation, however I don't know if any search engine that cares about <nav> yet.